The present invention relates to time counting circuits for measuring such time as the pulse spacing of a pulse signal, pulse converting circuits, and FM demodulating circuits for demodulating FM signals whose frequencies are modulated.
It has been expected that time counting circuits for measuring such time as the pulse spacing of a pulse signal are utilized in the field of digital communication and the like. In recent years, it has become possible to mount a time counting circuit composed of a CMOS transistor in conjunction with other digital circuits on a single chip, which has accomplished a significant reduction in the cost of manufacturing semiconductor devices including the time counting circuits.
A time counting circuit which exhibits further stabilized operation with higher accuracy may also find extensive application in various fields including the demodulation of FM signals and the demodulation of bus signals in an LSI. If a time counting circuit capable of measuring an extremely short time with accuracy and stability can be utilized for the demodulation of FM signals, the cost of a semiconductor device can be reduced significantly.
FIG. 10 is a block diagram showing an example of the structure of a conventional time counting circuit. The time counting circuit shown in FIG. 10 obtains time data which represents the pulse spacing of pulse signals to be measured which are inputted to an input terminal 66, and outputs the same time data from an output terminal 67 (see "Time/Numeric Value Converter LSI" Technical Report of IEICE, ICD93-77 (1993-08)).
In FIG. 10, the reference numeral 61 designates an inverter ring in which 33 inverters are connected in a ring. Since an odd number of inverters are connected in a ring, the inverter ring 61 oscillates so that the signal transition from the HIGH level to the LOW level or from the LOW level to the HIGH level circulates on the inverter ring 61 in a constant period. The reference numeral 62 designates a signal processing circuit which inputs the output signal of each inverter composing the inverter ring 61 and generates numeric data in accordance with the current position of the signal transition. The reference numeral 63 designates a counter which counts the change of the output signal of the 33rd inverter. The count data indicates the circulation number of the signal transition in the inverter ring 61. The reference numeral 64 designates a row of FFs (flip-flops) which holds and outputs the count data outputted from the counter 63 when a pulse signal to be measured which is inputted from the input terminal 66 rises. The signal processing circuit 62 also outputs the generated numeric data when the pulse signal to be measured rises. The reference numeral 65 designates an operating circuit which outputs time data indicative of the pulse spacing of the pulse signal to be measured from the output terminal 67 based on the numeric data outputted from the signal processing circuit 62 and the count data outputted from the row of FFs 64.
However, the conventional time counting circuit has the following problems.
In the conventional time counting circuit, the signal processing circuit 62 outputs the numeric data with the rising timing of the pulse signal to be measured, and the row of FFs 64 also outputs the count data of the counter 63 with the rising timing of the pulse signal to be measured. For this reason, the time data outputted from the output terminal 67 represents the time taken from the rise of the pulse signal to be measured to the next rise thereof.
For example, it is supposed that an FM signal is demodulated by the conventional time counting circuit. The FM signal oscillates with a constant amplitude and its frequency changes with time. A frequency for each time is information. In this case, the FM signal is first converted to an FM pulse signal. The FM pulse signal is obtained by amplifying the FM signal to a source voltage and a ground voltage with a reference voltage as a center. The time taken from the rise of the FM pulse signal to the next rise thereof (which is a period of the FM signal) is obtained by the time counting circuit. The frequency of the FM signal for each time can be obtained by getting the inverse number of the period.
However, the frequency of the FM signal also changes in one period. For this reason, there is a possibility that sufficient demodulation accuracy cannot be obtained by demodulation on such a period basis. For example, in the case where a voice is converted to the FM signal to be transmitted and demodulated by the conventional time counting circuit, the distortion of the signal waveform is caused so that the reproduction accuracy of the FM signal is deteriorated. Consequently, it is apparent that the original voice which is clear cannot be reproduced.
If the time taken from the rise to the fall of the FM pulse signal and the time taken from the fall to the rise thereof can be measured, the reproduction accuracy of the FM signal can be enhanced so that a clear voice can be reproduced. However, the conventional time counting circuit has a problem that the time taken from the rise to the fall of the pulse signal or the time taken from the fall to the rise thereof cannot be measured.
While there are some methods for improving the conventional time counting circuit so as to pick up the rising and falling edges of the pulse signal to be measured together, it is very difficult to cause the time to pick up the rising edge to coincide with the time to pick up the falling edge.